High-frequency oven for high powers



2, 6 H. L. VAN DER HORST ETAL HIGH-FREQUENCY OVEN FOR HIGH POWERS Filed June 9, 1958 mvsuToRa- HAJO LORENS VAN DER ORST PETER HUBERTUS GERARDUS VAN VLODROP AGENT United States Patent ice 3,015,783 HIGH-FREQUENCY OVEN FOR HIGH POWERS Hajo Lorens van der Horst and Peter Hubertus Gerardus van Vlodrop, Eindhoven, Netherlands, assignors to North American Philips Company, Inc., New York,

N.Y., a corporation'of Delaware 7 I Filed June 9,v 1958, Ser. No. 740,717

Claims priority, application Netherlands July 13, 1957 The present-invention. relates to high frequency. oven circuit arrangements. More particularly, the invention relates to high frequency oven circuit arrangements for high powers, for example several tens of kilowatts, having a gaseous discharge tube for exciting an oscillatory circuit, the gaseous discharge tube having a control electrode which, when oscillations of considerable amplitude are still present in the oscillatory circuit of the high-fre quency oven circuit arrangement, has supplied to it periodical ignition pulses derived from a separate relaxation generator having a grid-controlled discharge tube.

The gaseous discharge tube used in such high-frequency ovencircuit arrangement may. bev of difierent types, for example gaseous discharge tubes having a thermionic cathode. and a grid-like control'electrode, or gaseous dis charge tubes having a mercurypool cathode and a control electrode or ignition electrode arranged therein in an insulated manner.

In accordance with the invention, the control-grid circuit of the discharge tube is connected in a relaxation.

generator circuit. A sawtooth voltage provided by the relaxation circuit and a sinusoidal voltage provided by the oscillatory circuit of the high-frequency oven circuit ar-;

rangement are supplied to the control grid of the discharge tube for producing ignition pulses. .The ignition pulses cause ignition of the gaseous discharge tube each time the anode voltage of the gaseous discharge tube constituted by the direct anode voltage and the superim posed alternating voltage of the circuit .has in practice a minimum value.

The application of the steps according to the invention results in an improvement in the efiiciency, of operation of the high frequency oven circuit arrangement and the components of said circuit arrangement have relatively small ratings.

In order that the invention may be readily carried into efiect, one embodiment will now be described in detail, by way of example, with reference to the accompanying drawing in which FIG. 1 shows a high-frequency oven according to the invention and FIGS. 2a-2c show several time diagrams toexplain the performance of the high-frequency oven of FIG. 1.

The high-frequency oven shown in FIG. 1, which is designed, for example, for a power of kw., has a discharge tube 1 filled with hydrogen gas, for exciting an oscillatory circuit 2 constituted by a circuit capacitor 3 and a circuit coil 4. The circuit coil 4 is coupled to a coupling coil 5, which is connected to a work-coil 6 containing a load 7.

Connected in series with-the oscillatory circuit 2 is a charging capacitor 8, which is connected through a choking coil 9 to a direct-current source 10 constituted, for example, by a three phase rectifier, a coil 11 being included between the charging capacitor 8 and the anode of tube 1. A

The gaseous discharge tube 1 has a control electrode which, when oscillations of considerable amplitude are still present in oscillatory circuit 2, has supplied to it periodical ignition pulses provided by a separate relaxation generator. In the embodiment shown, the relaxation generator comprises a thyratron 13 which normally is cut ofiby a negative grid-bias source 12. The anode circuit cludes the series-combination of a resistor 16 and a coil 7 17. The relaxation capacitor is charged across resistor 14 during each cycle at. a charging speed determined by the time constant of resistor 14 and capacitor 15. A

" "f'Ihe relaxation generator sawtoothvoltage set up across capacitor 151being applied to thecontrol electrode of-thyratronglii through an adjustable voltage divider 23 included between the anode and the negative. terminal of the grid-bias source 12."

Upon every ignition of thyratron 13, the relaxation capfacitor 15 is discharged, resulting in a positive pulse across the cathode impedance 16, 17of said thyratron. The thyratron 13 produced pulse is supplied through a blocking capacitor 18 to the control grid of a thyratron 1:9,is connected as a pulse producer. The thyratron '19, which normally is cut-off by a negative grid-bias provided by grid-voltage source 12, includes a delay line 20 in its anode circuit, ,its cathode circuit including a resistor 21. When a positive pulse occurs ,at-the control grid or thyratron 19, the delay line 20 discharges through thyratron l9'and across the. cathode resistor 21 thereis setup a positive rectangularpulse which is supplied as an ignition pulse through resistor- 22 to the control electrode o gaseous discharge tube 1.-

for producing the ignition pulses is designed sotliat the vcontrol gridof thyratron 13 has also suppliedto it a sinusoidal voltage provided by the oscillatory circuit 2 of the high-frequency oven circuit arrangement. The sinusoidal voltageis derived from a coil 24 coupled to the oscillatory circuit 2. Each time the sumof the sawtooth voltage and the sinusoidal voltage exceeds a threshold value formed by the negative grid-bias of thyratron 13, an ignition pulse is produced, which occurs at amornent when the anode voltage of the gaseous discharge tube 1, which is constituted by the direct anode voltage and the superimposed alternating voltage of the circuit,has a minimum value. p

. The performance of the described high-frequency oven will now be explained with reference to the time diagrams shownin FIG. 2, wherein we assume that the high-frequency .oven circuit arrangement is initially loaded by a work-piece 7.

FIG. 2a shows the voltage at the anode of gaseous discharge tube 1, which is constituted by the direct voltage across chargingcapacitor 8 as represented by the dotted curve 25and the superimposed voltage 26 of the circuit.

At the moment .t when the voltage at the oscillatory circuit 2 has a negative peak value, that is to .say, the anode voltage is minimum, the gaseous discharge tube 1 is ignited by an ignition pulse, resulting in the circuit capacitor 3 being charged by the charging capacitor 8 via the series-coil 11 until tube 1 is extinguished at the moment when the anode voltage of gaseous discharge tube.

1 passes through zero. g I

The energy in the oscillatory circuit 2 can now swing out freely, resulting ina damped oscillation 26, while charging capacitor 8 is charged by the direct-current source 10 via choke coil 9 in accordance with curve 25.

- At the time t which corresponds-to a minimum anode voltage of gaseous discharge tube 1, a subsequent ignition pulse is supplied to the control electrode of gaseous discharge tube 1, so that the circuit capacitor 3 is charged to its initial maximum value and the above-described process is repeated.

Each time an ignition pulse occurs which coincides with a negative peak value of the oscillations set up in the oscillatory circuit 2, that is when the anode voltage of gaseous discharge tube 1 is minimum, a charging .cur-

charging capacitor 8 and coil 11, which charging current Patented Jan. 2, 1962- is proportional to the instantaneous anode voltage and may be several hundreds or thousands of amperes. The value of this charging current is dependent upon the amplitude of the sinusoidal oscillation in the oscillatory circuit 2 upon occurrence of an ignition pulse, which amplitude is determined by the load on the high-frequency'oven, said charging current increasing with increasing load on the high-frequency oven.

By using the step according to the invention, it is ensured that the circuit capacitor 3 isrecharged to its initial maximum value with minimum charge shift without any interfering building-up phenomena and it has been found that a considerable improvement in efficiency is thus obtained, for example an efliciency of more than70%. A further advantage of the indicated step is that the elements of the high-frequency oven may be proportioned for a lower power. j

FIG. 2b shows the grid voltage of the thyratron 13, which is connected as a relaxation generator, said grid voltage being constituted by a sawtooth voltage 27 with. the sinusoidal voltage 28 derived from oscillatory circuit 2 superimposed thereon. Each time this grid voltage exceeds'the threshold value 29 determined by the block ing voltage of thyratron 13, the thyratron 13 is'ignited' and an ignition pulse occurs, which, as may be seen from the figure, coincides with a minimum value of the anode;

voltage of gaseous discharge tube 1'.

FIG. 20 shows the ignition pulses produced. In the high-frequency oven shown in'FIG. 1, the recurrence' frequency of the ignition pulses is, for example, 250 c./sec. and the frequency of the oscillations occurring in oscillatory circuit 2 is 10 kc./ sec.

The useful output of the described high-frequency oven may be adjusted within very wide limits, for example, from about 0 kw. to about 7 kw., in a particularly simple manner by varying the recurrence frequency of the igni-1 tion pulses. This is effected by adjusting the adjustable voltage divider 23 included in the control-grid circuit of thyratron 13. The ignition of the gaseous discharge tube 1 always takes place at a minimum anode voltage of gaseous discharge tube 1, thus resulting in a favourable output. i

If the described high-frequency oven is used for heating work-pieces of diiferent size, or if variations in'l'oad occur during the heating process, the ignition frequency may vary due to the amplitude of the sinusoidal voltage applied to the control grid varying with the load. For example, when the load onthe high-frequency oven is small, the amplitude of the sinusoidal voltage has a high value and the thyratron 13 can ignite at a preceding peak of thesinusoidal voltage. This results in an increased recurrence frequency of the ignition pulses,.which under certain conditions may be objectionable in connection with the useful output. For this purpose, the sinusoidal voltage of oscillatory circuit 2, which is applied to the control grid of thyratron 13, is also applied via a resistor 30 to a rectifier stage 31, which provides a negative voltage in series with the voltage of the grid-bias source 12. The blocking voltage of the thyratron thus varies with the mean amplitude of the oscillations occurring in oscillatory circuit 2, with the result that variations in the recurrence frequency of thee ignition pulses are counteracted or avoided.

Of a high-frequency oven of the specified type which has been extensively tested in practice, the following data are mentioned below:

What.is claimed is: 1. A high frequency circuit arrangement compr sing for. deriving an auxiliary substantially sinusoidal voltage.

a gaseous discharge tube having an anode and a control electrode, means for supplying a direct voltage to said anode, a resonant circuit connected to said anode, a load circuit connected to said resonant circuit, means for periodically initiating the operation of said discharge tube thereby to periodically energize said resonant circuit, said last-mentioned means comprising an electrical discharge device having a control electrode and an output electrode, means for applying a signal having a substantially sawtooth waveform to said control electrode, means for deriving an auxiliary voltage from said resonant circuit and for superimposing the'same on said signal, and means for connecting said output electrode to the control electrode of said gaseousdischarge tube.

2. A high frequency circuit arrangement comprising ;a gaseous discharge tube having an anode andv a control electrode, means for supplying a direct voltage to said anode comprising an energy storing capacitor, a resonant circuitconnected to said anode and said capacitor in series circuit arrangement, a load circuit connected to said resonant circuit, means for periodically initiating the operation of said discharge tube thereby to periodically discharge said capacitor through said resonant circuit,v

said last-mentioned means comprising an electrical discharge device having a control electrode and an output electrode, means for applying a signal having a substantially sawtooth waveform to'said control electrode, means from said resonant circuit'and for superimposing the same on said signal, and means connecting said output electrodeto the control electrode of said gaseous discharge tube.

3. A high frequency circuit arrangement comprising a gaseous discharge tube having an anode and a control electrode,.input means for a direct voltage source, an

inductor interconnecting said input means and said anode, an energy storing-capacitor, a resonant circuit connected.

to said anode and said capacitor in series circuit arrangement, a. load circuit connected, to, said resonant circuit,

means for periodically initiating the operation of said discharge tube thereby to periodically discharge said capacitor through said resonant circuit and energize said load circuit, said last-mentioned means comprising a gaseous discharge tube having a control electrode, a capacitor and an impedance connected to form a relaxation oscillator generating a first signal having a substantially sawtooth waveform and a second signal having a pulse waveform, means for applying said first signal to said last-mentioned control electrode, means for deriving an auxiliary voltage from said resonant circuit and for superimposing the same on said first signal, and means responsive to said second signal for initiating the operation of said first-mentioned discharge tube.

4. A high frequency circuit arrangement comprising a gaseous discharge tube having an anode and a control electrode, means for supplying a direct voltage to said anode comprising an energy storing capacitor, a resonant circuit connected to said anode and said capacitor in series circuit arrangement, a load circuit connected to said resonant circuit, means for initiating the operation of said discharge tube thereby to periodically discharge said capacitor through said resonant circuit, said lastmentioned means comprising a gaseous discharge tube having a control electrode, a capacitor and an impedance connected to form a relaxation oscillator generating a first signal having a substantially sawtooth waveform and a second signal having a pulse waveform, means for applying said first signal to said last-mentioned control electrode, means for deriving an auxiliary voltage from said resonant circuit and for superimposing the same on said first signal, means for deriving from said auxiliary voltage a biasing voltage having variations as determined by the variations of said auxiliary voltage, means for applying said biasing voltage to said last-mentioned control electrode, and means responsive to said second signal 5 6 for initiating the operation of said first-mentioned dis- References Cited in the file of this patent marge P UNITED STATES PATENTS 5. A high frequency c1rcu1t arrangement as claimed in claim 4 further comprising means for adjusting the 2,492,161 F 1949 intensity of said first signal applied to the said second- 5 2,591,660 Lmdley et 1952 mentioned control electrode. 2,852,676 Joy Sept 1958 

